of Equality Formation from site 30643, associated with an aquatic gastropod assemblage. The overall assemblage is consistent with fluctuating water levels in a slackwater lake, an interpretation also noted by Shaw (1921) for deposits in glacial Lake Kaskaskia.
Fossil conifer wood and needles, probably of Picea sp., were encountered at a depth of 27 feet in stratigraphic test hole no. 30678 (near Fayetteville, cross section A–A9), below Cahokia Formation alluvium. Wood in the Equality Formation at this site was radiocarbon dated at 15,280 ± 400 14C years before present, confirming a last glacial age. Because the base of the unit was not encountered, it is suspected that the lake was present at considerably earlier times as well.
At some sites (including 30636 and 30694), the lower Equality Formation has a slight pinkish brown cast, is more clayey than above (a silty clay loam), and tends to be weakly calcareous or leached; fossil gastropods (Stagnicola caperata) are present at some sites. Small conifer wood fragments at the base of a 40-foot sequence of Equality Formation deposits at outcrop 30636 along Silver Creek were dated at 21,570 ± 160 14C years before present. This radiocarbon age likely represents a minimum age of initial aggradation for most of the Equality Formation in New Athens area terraces and corresponds to the maximum extent of the last glaciation in northeastern Illinois (Hansel and Johnson 1996) and in the Upper Midwest. Earlier periods of Equality Formation deposition likely occurred on more downstream reaches of the Kaskaskia River valley, at lower elevations, as found by Curry and Grimley (2006) on the edge of the American Bottoms near St. Louis.
Postglacial stream deposits in Silver Creek, Mud Creek, Doza Creek, and other tributary valleys are mainly fine grained (silty clay loam to silt loam) and weakly stratified. These deposits, mapped as Cahokia Formation, can include loamy zones or beds of fine sand. The Cahokia Formation in these valleys is less than 20 feet thick and consists mainly of reworked loess, lake deposits, and other fine-grained sediments. The fine-grained nature of these deposits can be explained by the rare occurrence of sand, gravel, or diamicton exposures in the creek watersheds. Due to periodic flooding during postglacial times, areas mapped as the Cahokia Formation have relatively youthful, modern soil profiles that generally lack B horizons compared with profiles for upland soil (Wallace 1978, Natural Resources Conservation Service 1999).
Kaskaskia River Valley
Near-surface deposits in the postglacial Kaskaskia River valley (~16% of map area) consist of interstratified fine to medium sand with silt loam, silty clay loam, and silty clay. Sandy deposits (up to 25 feet thick) in channels and point bars of the Kaskaskia River are mapped as sandy facies of the Cahokia. These deposits are typically fine to medium sand, moderately well sorted, and noncalcareous. They range in age from recent in modern point bars to possibly several thousand years old (mid to early Holocene) at higher elevations and in the subsurface. The clayey facies of the Cahokia Formation is divided into two units: c(c)-2, older deposits generally on low terraces between about 390 and 405 feet asl, and unit c(c)-1, younger deposits at lower elevations on the modern floodplain. Both deposits range from silt loam to silty clay loam to silty clay and are interpreted mainly as overbank flood deposits and swale fills. Deposits of c(c)-2 are relatively thin (5 to 20 feet thick) and overlie Cahokia sandy facies or the last glacial deposits of the Equality or Henry Formations. Aerial photographs show many abandoned meander channels within the modern floodplain, all now infilled with clayey sediment c(c)-1, up to about 20 feet thick. Both c(c)-2 and c(c)-1 are interstratified laterally with the Cahokia sandy facies. Numerous archeological sites with projectile points, tools, and fire-cracked rocks have been noted in the lower Kaskaskia River floodplain (Conrad 1966), including areas in this quadrangle mapped as Cahokia Formation (clayey or sandy facies).
In many areas in the Kaskaskia River valley, fine to medium sand deposits immediately below the Cahokia and/or Equality Formations are interpreted as Henry Formation (typically Wisconsin Episode age), although the exact age of these deposits are unknown and may include Sangamon Episode fluvial deposits. As the distinction among alluvial units can be subtle and difficult to differentiate, areas noted as Henry Formation in cross section may include sandy Cahokia Formation in upper portions or Pearl Formation near the unit base. In general, the Pearl Formation tends to contain more coarse sand and gravel than the Henry Formation in the Kaskaskia River valley area (probably due to closer proximity to the ice margin), but there may be considerable overlap in the grain size of these units. Sand in the Henry Formation can be noncalcareous or calcareous and may be intercalated with or overlain by calcareous silt loam beds of the Equality Formation.
In some areas, the Cahokia or Equality Formations may directly overlie the older Pearl Formation where the Henry Formation was eroded or was absent (see cross section A–A9). The Illinois Episode Pearl Formation is of significant thickness in the Kaskaskia River valley (up to 55 feet thick) and occurs below the younger river or lake deposits. The Pearl Formation could possibly include pre-Illinois Episode fluvial deposits that cannot be differentiated based on limited data. Several coal and oil borings (not shown on map) were drilled in the Kaskaskia River valley in this quadrangle, but provide only very minimal descriptions of unconsolidated surficial materials in the subsurface. In sum, the Kaskaskia River valley contains a complex record of fluvial and glacial deposits from perhaps the past 500,000 years, all overlying Paleozoic bedrock.
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